An excavator could be seen digging through dirt at a site surveyed by investigators last September.

The new effort was announced Monday, and is the result of a multi-agency investigation -- along with information from convicted killer Wesley Shermantine.

The FBI gave an update on its plans to excavate a wellsite in Central California linked to two serial murderers known as the Speed Freak Killers.

Shermantine is among the sources used in the investigation, FBI officials said in a news release.

The FBI took over responsibility for the excavation from the San Joaquin County Sheriff's Department, which was criticized for its handling of the operation.

The well has been linked to Shermantine and Loren Herzog, the so-called Speed Freak Killers who authorities say went on a methamphetamine-fueled killing spree in the 1980s and '90s.

"We all remain hopeful that our efforts at this site will ultimately return the remains of victims to their loved ones but know that such is not a certainty," said Herbert H. Brown, a special agent in charge of the Sacramento Division of the FBI.

The FBI is not certain who, if anyone, may be found at the new location.